The electrical characteristics of a number of electronic components are entirely defined by their geometry in a plane. This is the case with transistors for example. This characteristic affords a high degree of manufacturing versatility, allowing components with very different levels of performance to be combined on one and the same chip. This is not the case however with bulk acoustic wave (BAW) components, the electrical characteristics of which are entirely defined by the nature and the thickness of the stacks of component layers. Even for surface acoustic wave components (SAW), in which case the electrical characteristics are largely defined by the patterns of the electrodes used, the properties of the waves employed have proven to be sensitive to the ratio between the thickness of these electrodes and the wavelength, and hence the frequency. As a result, each filter is currently produced on an individual chip, thereby necessitating a corresponding number of filter chips to be assembled (a modern mobile telephone generally uses about 40 different filters), and requiring complex assembly diagrams.
In order to overcome this issue, multiple solutions have been proposed in order to allow multiple filters to be co-integrated on one and the same chip. Often, the proposed embodiments involve the simultaneous production of the two filters of a duplexer. In this case, the separation between the two target passband bandwidths is actually generally relatively small, and co-integration is justified by the fact that the assembly formed by the two filters must be added to a substrate comprising passive elements allowing transmission and reception channels of the duplexer to be electrically isolated from one another.
In the case of SAW components, the resonator resonant frequency is defined by the period of metallizations taking the form of interdigitated combs which are used to excite the acoustic waves on the surface of piezoelectric substrates. The excitation of the acoustic waves occurs specifically when the synchronism condition p=λ/2=V/(2f) (where p is the period of the interdigitated electrodes, λ is the wavelength, V is the speed of propagation of the surface wave and f is the frequency) is satisfied.
In order to produce filters at different frequencies, those skilled in the art will therefore use different electrode periods in order to position the various components at different frequencies. Use will also be made of the anisotropy of the piezoelectric substrates employed in order to benefit from optimal propagation conditions for the purpose of fixing the electromechanical coupling coefficients of the resonators (which defines the bandwidth of each filter) and their temperature drift. Nevertheless, all of these properties are also heavily influenced by the ratio between the thickness of the metallizations and the wavelength, the presence of a metal layer making the surface waves dispersive. As long as the spacing in terms of frequency between the various filters to be produced is relatively small, these frequency dispersion effects remain negligible. It is for this reason that some duplexers are based on two filters formed conjointly on one and the same chip. However, if the various filters that it is sought to produce have a substantial frequency coverage, these effects dominate and may therefore affect the performance of some of the filters to be produced.
Regarding bulk acoustic wave filters (BAW), a first solution proposed in the literature consists of using a reference filter, generally that which will have the highest center frequency. This filter is made from two sets of bulk wave resonators having slightly offset resonant frequencies, thereby allowing a bandpass circuit to be synthesized on the basis of ladder or lattice filter topologies. In a known manner, the frequency offset between these two types of resonators is obtained by adding an additional layer, referred to as a mass overload, to the resonators located on the parallel branches of the circuits in order to lower their resonant frequency with respect to the resonators on the series branches. The second filter will subsequently be produced by using the same principle of lowering the frequency, i.e. by adding an additional layer localized below the filter covering the lowest frequency band. This principle is illustrated in FIG. 1.
This solution was initially proposed by Lakin in 2000 in the article: K. M. Lakin, K. T. McCarron, J. Belsick and R. Rose, Filter banks implemented with integrated thin film resonators, Proceedings of the 2000 IEEE Ultrasonics Symposium, pp. 851-854 (2000) and it was experimentally demonstrated in 2008 by Nam and described in the article: Nam et al., Monolithic 1-chip FBAR duplexer for W-CDMA handsets, Sensors and Actuators A, vol. 143, pp. 162-169 (2008), in which the reception and transmission filters of a duplexer for the WCDMA standard were simultaneously produced. This demonstration uses filters of FBAR (film bulk acoustic resonator) type, but its principle may be directly applied to resonators of SMR (solidly mounted resonator) type.
In order to produce the two filters of a duplexer at the same time, it is in theory necessary to use three mass overload layers, since ultimately a set of four resonators, offset by frequency, must be produced. In their work, the authors of this publication were able to optimize the design of the stacks in order to be able to decrease the number of mass overload layers and thus simplify production. Thus, the stack of the transmission filter differs from that of the reception filter only in the local addition of a “band offset” layer (“band tuning layer” in the original publication). This is illustrated in FIGS. 2a to 2d, which relate to an exemplary embodiment of the two filters of a WCDMA duplexer in the article by Nam et al., Monolithic 1-chip FBAR duplexer for W-CDMA handsets, Sensors and Actuators A, vol. 143, pp. 162-169 (2008).
The advantage of this solution is that the extra technological cost associated with its production consists only of a photolithography mask level and a deposition and structuring step. This solution therefore remains very simple to implement. Nevertheless, the addition of an additional layer to certain resonators leads in practice to a decrease in the electromechanical coupling coefficient, which may result in additional insertion losses and/or an undulation of the transmission in the target frequency band, and/or in a decrease in the effective passband bandwidth of the filter thus produced, thereby negatively affecting the performance of the component. This negative effect worsens as the frequency gap between the filters widens, given that a more substantial mass overload must be used in this case. In practice therefore, this solution may be employed only in configurations in which the target frequency coverage is not very wide. In the case of the WCDMA duplexer presented in the reference by Nam et al. mentioned above, the two filters have a relative passband bandwidth of about 3%, and are spaced apart in terms of frequency by 9%. The FBAR structure used is able to tolerate these constraints. Nevertheless, numerous other radiofrequency bands prove to be more problematic.
Teams at the VTT have also demonstrated that effectively covering multiple bands using BAW components involves different thicknesses of piezoelectric material for each of the filters, as described in J. Ellä and H. Pohjonen, BAW filters having different center frequencies on a single substrate and a method for providing the same, patent U.S. Pat. No. 6,518,860B2. The constraints on filter synthesis are effectively such that the piezoelectric material used to produce the bulk wave resonators, aluminum nitride, only just has sufficient piezoelectric properties to satisfy them. Regarding dimensions, everything therefore occurs as if it were sought to produce each filter on a different chip. When it comes to manufacture, things get more complex.
A first approach proposed in the patent in question consists of depositing a first layer of piezoelectric material at a thickness corresponding to the smallest of the required thicknesses of material. This layer will subsequently be protected at the places where the filter requiring this thickness will be localized, generally the filter with the highest frequency. Next, another deposition of the piezoelectric material is carried out, resulting in this layer being thickened at the places where it is not protected, i.e. where the filter with the lowest frequency is located.
The feasibility of this approach has been studied by a team at the Swiss Federal Institute of Technology in Lausanne, described in: F. Martin, P. Muralt, M. Cantoni and M. A. Dubois, Re-growth of c-axis oriented AlN thin films, Proceedings of the 2004 IEEE Ultrasonics Symposium, pp. 169-172 (2004).
The authors have shown that in practice, this approach of regrowing the layer leads to a decrease in its piezoelectric properties with respect to a direct deposition of an equivalent thickness. They bring to light that this is linked to the fact that, in the case of aluminum nitride (AlN), the added extra thickness exhibits poor crystallinity, or even an inversion of its polarity, thereby leading to a partial nullification of the piezoelectric effect produced by the original layer. This behavior is attributed to a degradation in the surface of the AlN layer caused by the chemical agents used in the cleanroom fabrication process steps (photolithography in particular), which are known for lightly etching the material and for increasing its roughness. This phenomenon is also visible after simply exposing the material to the air after deposition, although the effect is less substantial in this case. This indicates that a simple oxidation or a reaction with the humidity in the air is enough to cause a degradation in the surface and to prevent regrowth that preserves the polarity of the material. This study therefore shows that such an approach, at least in the case of AlN, is not viable.
Those skilled in the art could envisage taking the opposite course to the preceding approach, and envisage depositing the thickest piezoelectric layer first, then locally etching it in order to thin it down to the thickness desired for the filters with the highest frequencies. This has the advantage of preserving the structural integrity of the material. Nevertheless, this approach comes with two critical flaws:
the etching may lead to a roughening of the partially etched layer, which results in a decrease in the quality coefficient of the resonators. Worse, in the case of resonators made of aluminum nitride, the chemical products used for the photolithography and resist removal steps also lead to the state of the surface of the material being degraded;
however, the resonators are very sensitive to the thickness of the piezoelectric layer. Great progress has been made in bringing the techniques of depositing thin piezoelectric (in particular AlN) layers to a satisfactory level in terms of controlling the thickness (currently reaching a level of uncertainty as to the deposited thickness corresponding to a standard deviation of the order of 0.3% of the target thickness). In an approach consisting of locally thinning the piezoelectric layer, the residual dispersion of thickness adds the uncertainties in thickness that are linked to the deposition of the layer to the dispersions due to etching, which are generally relatively substantial, especially in the case of partial etching without a stop layer. Such an approach would therefore lead to substantial frequency dispersions, which would result in drops in manufacturing yield.
In order to overcome the limitations of these latter two approaches, the Technical University of Madrid has proposed an approach that can be applied to AlN BAW resonators by making use of the technological particularities of the stacks commonly used, as described in the international patent application by E. Iborra, M. Clement, J. Olivares, Device for filtering bulk acoustic waves, WO2010/116011 A1 (2010).
The principle of this approach consists of first depositing a first piezoelectric layer of aluminum nitride, then of covering it with a layer acting as a hard mask. This may for example be the metal layer forming the upper electrode. This hard mask will subsequently be defined in order to remove it from the places where it will be desired to produce the other series of filters. The aluminum nitride will subsequently be etched in order to leave only islands on which the first series of filters will be produced. A second layer of aluminum nitride will subsequently be deposited, aiming for a thickness corresponding to the second series of filters. This second layer is once more covered with a hard mask, which is in turn defined in order to clear the spaces where there will be no need for the second thickness of piezoelectric material. The second layer of aluminum nitride is then etched. At this stage, the etch will abut either the first hard mask, thereby allowing the second layer of AlN to be removed and the remaining island of the first layer to be protected; or the second hard mask, thereby protecting the second layer of AlN, which has been deposited on an area from which the first layer has already been removed beforehand. Islands having different thicknesses are thus formed. The process may then be repeated over and over again. It is then sufficient to etch the hard masks by making use of the selectivity of etching between these masks and AlN, thereby ensuring that the piezoelectric layers will not be degraded. In the case in which this hard mask is made from metal layers forming the upper electrodes (made of Mo in the patent in question), this etching step may be directly used to form the patterns of the upper electrodes. If this is done by means of reactive dry etching using fluorinated gases, the method is extremely selective with respect to the aluminum nitride layer, thereby allowing different thicknesses to be accommodated on each of the islands without affecting the production process.
This approach has been experimentally validated by the applicant and described in the article: A. Reinhardt, J. B. David, C. Fuchs, M. Clement, J. Olivares, E. Iborra, N. Rimmer, S. Burgess, Multiple-frequency solidly mounted BAW filters, Proceedings of the 2011 International Frequency Control Symposium, shortly afterward by taking the case of the two filters of a duplexer for the WCDMA standard. This article shows a cross section of two stacks produced for the various filters, which differ in the thickness of the aluminum nitride layer, namely 1.29 μm for the transmission filter and 1.11 μm for the reception filter. This article also shows the responses of the two conjointly produced filters, as well as the validity of the approach as reprised and illustrated in FIG. 3.
This approach also has a few limitations, however:
it requires a certain number of additional fabrication steps in order to form the islands of piezoelectric layers having different thicknesses. It therefore incurs extra cost to produce which is to be compared with the extra cost of assembling two different chips on one and the same module instead of only one containing the two co-integrated filters;
furthermore, in order to limit the technological cost of this approach, technological stacks should be set up for each filter while attempting to preserve a maximum number of layers of identical thickness between the various filters. For performance-critical applications, technologies based on aluminum nitride may not be capable of affording the designer sufficient freedom to permit such a sharing of layers between various filters, since these properties are only just capable of meeting the specifications of the existing filters.
In view of this observation and faced with the increased number of filters in mobile communication systems, a need remains to identify a technology capable of synthesizing the majority of the bandpass filters required for these systems to operate, allowing a maximum number thereof to be co-integrated on one and the same chip, and ideally without extra technological cost.
One solution to this problem has been indirectly provided by the team of Pr. Hashimoto at the University of Chiba. This team has for several years specifically been interested in the use of resonators with very high electromechanical coupling coefficients for the purpose of producing frequency-agile filters: K. Y. Hashimoto, S. Tanaka, M. Esashi, Tunable RF SAW/BAW filters: dream or reality?, 2011 Joint Conference of the IEEE International Frequency Control Symposium and the European Frequency and Time Forum. Specifically, the addition of variable passive elements in series or in parallel with an acoustic resonator allows the resonant and antiresonant frequencies to be shifted as illustrated in FIGS. 4a and 4b. 
It is therefore possible to substantially shift the resonant and antiresonant frequencies of a resonator within the limits of the range delimited by the original resonant and antiresonant frequencies, without the capacitive adjustment elements. A very high electromechanical coupling coefficient becomes the necessary condition for very high frequency agility. In order to make use of this principle, filter topologies such as for example those presented in FIG. 5 are used. By deciding to employ SAW resonators having the highest possible electromechanical coupling coefficients (higher than 30% in this instance), these authors have succeeded in designing a filter circuit having a relative bandwidth of 5%, the center frequency of which is capable of varying by 5%. In order to demonstrate this variable filter principle, in practice the authors produced two separate filters having identical resonators and different capacitance values as described in: T. Komatsu, K. Y. Hashimoto, T. Omori, M. Yamaguchi, Tunable radio-frequency filters using acoustic wave resonators and variable capacitors, Japanese Journal of Applied Physics 49, 2010 instead of directly producing a complete frequency-agile filter module. Here they have proposed, in a fortuitous manner, a solution allowing two filters operating at different frequencies to be produced on one and the same chip.
Varying the center frequency of a filter according to this approach, even if it is operable, nonetheless meets with a strict limit. Specifically, the authors at the University of Chiba present, as illustrated in FIG. 6, two filters with 5% relative bandwidth, centered at 4 and 9% of a reference frequency. Nevertheless, having based their filter on resonators having an electromechanical coupling coefficient of 30%, a much higher frequency agility, or else a much greater spacing in terms of frequency between the two filters, could be expected.
In practice, in the approach proposed by the team at the University of Chiba, the frequency variation afforded by modifying the capacitance values of the capacitors inserted into the filter circuit is limited by the fact that the characteristic impedance of the resonators is modified by the capacitance value of the capacitors connected in series and in parallel with the resonators, as can be seen in FIGS. 4a and 4b. Thus, as soon as the values of the passive elements are varied substantially for the purpose of increasing the spacing in terms of frequency between the two filters, an electrical mismatch of the filter is very quickly entrained, thereby making it non-operational. This effect is illustrated in FIG. 7, which shows the deterioration in the response of a filter as it is sought to shift its frequency by varying the capacitance values of the capacitors inserted into the filter circuit.